Inhibition of corrosion



United States Patent rNrmrrroN 0F CORROSION James E. Atherton, .lr., Catawissa, Pa., and David H. Gurinsky, Center Moriches, N. Y., assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Application May 26, 1955 Serial No. 511,442

7 Claims. (Cl. 75134) The present invention relates to a method for, preventing corrosion of container materials by liquid metal compositions containing bismuth.

Bismuth has been found to be one of the metals having properties suitable for transferring heat produced by nuclear reactors. It has also been found suitable as a solvent and dispersion medium for fissile and fertile metals, such as thorium, uranium and plutonium, used in connection with nuclear reactors. In order to make use of bismuth in connection with nuclear reactors it must be contained in metal apparatus in which it can beflowed at high temperatures. However it has been found that liquid bismuth corrodes most metal container materials, particularly at high temperatures. One of the corrosive efiects which has been found to cause particularly harmful damage to steel containers is the tendency of bismuth to dissolve container metal in the higher temperature r 2,840,467 Patented June 24,

is to be contained, as for example in a heat transfer sys-, tem, the use of steels contain from 2 .to ;12%-chrornium and more than 0.5% of silicon or molybdenum is pre' dition when the bismuth contains dissolveduranium, be-

cause these metals lower the solubility of uranium in bismuth. The use of steels containing a minimum of the nickel and manganese components is therefore preferred.

Bismuth can be circulated isothermally in contact with the prefered steels without the use of inhibitors for relatively long periods of time at low temperatures, i. e., near the melting point of bismuth. The use of corrosion in-' hibitors becomes necessary when the bismuth is to be maintained in a container at relatively high temperatures. in the order of 400 C. and higher, andwhen -a temperature gradient exists inthe flow system. The particular corrosion inhibitor selected, and the concentration at which it is used, depend on .the metal content of -the sysportions of flow systems and to deposit it in the lower the flow system by deposit of metal in the colder sections.

This transfer of container metal is known as mass transfer corrosion.

Although there are some metals which are not corroded or attacked by bismuth there are few which have the other requisite properties for use in making the component parts of reactors which contain liquid bismuth. Such properties as structural strength, resistance to corrosion by air at elevated temperatures, and relatively low neutron capture cross section, are among the properties such a container metal should have. Materials which may be found to have the desired combination of properties for such use are prohibitively expensive. Certain steels available in commerce could be used advantageously in providing containers for liquid bismuth if they were not corroded by the bismuth.

It is accordingly one of the objects of the present invention to provide suitable metal containers for high temperature liquid bismuth. It is another object to provide a method of inhibiting the corrosion of metalsby bismuth at high temperatures. It is a further object to make possible the use of commercially available relatively low cost container materials having the best combination of structural, nuclear, thermal and other properties, in the fabrication of vessels for containing liquid bismuth at high temperatures. Other objects will be in part obvious and in part pointed out hereinafter.

In one of its broader aspects the objects of the present invention are achieved by providing containers formed from steel containing chromium, silicon and molybdenum in certain proportions and by incorporating zirconium and magnesium in the liquid bismuth which contacts the containers.

bismuth depends on the content of the bismuth to be brought into contact with the'ste el. Where pure bismuth inhibitors. Theseexamples also ill tem and the temperature gradient of the system. Where a temperature gradientof up to 150 C. exists in'the steel container system, the minimum temperature of which is 400 C., corrosion of the steels containing the indicated quantities of chromium and silicon or molybdenum, can. be inhibited by incorporating zirconium metal in the bis-v muth at a concentration of between 50 parts per. million and thesaturation concentration of zirconium at the lowest: temperature of the system. Where the lowest temperature of the system is approximately 400' a zirconium concen-' tration of between 200 and 250 parts per million has been found to provide effective inhibition. I

Corrosive attack of the indicated container materials can also be effectively inhibited'by the use of magnesium and zirconium in combination. For example, where a relatively high temperature differential, up to about 150 C., exists in the circulation system, a concentration of 100 parts per million both of zirconium and of magnesium effectively inhibits corrosion, although additions of higherconcentration of one. ,give' satisfactory inhibition with smaller concentrations of the other.

' When it is desirable to include a fissionable or fertile metal in the bismuth, such as uranium-235 or uranium- 238 respectively, and contain the composition in metal vessels, the use of higher concentrations of the corrosion inhibitors is desirable to avoid not only the corrosion of the vessel walls but also the loss from the bismuth of the fertile of fissionable materiaL' In this connection the inclusion of magnesium is particularly desirable to pre- Y satisfactory, both to inhibit corrosion of the indicated container metals and to prevent appreciable loss of uranium from bismuth.

and 250 parts per million ofmagnesium and zirconium have been found to provide effective inhibition.

The following are examples of the results 'o'f the circulation of liquid bismuth in contact with the indicatedcontainer materials, both with and without-the aid of the we e e. We

In systems, the lowest temperature of Y which is about 400 C., concentrations of between' 200 a,. PA.

use of lower concentrations of the other.

In each example tubular steel of the specified diameter, and metal' 'c'ontent -"was chemically cleaned by nitric acid solution (3 parts concentrated acid to 1 part water), thoroughly rinsed with water and methyl-alcohol and dried with filtered air. The tubing was formed into a loop through which the liquid metalcould be circulated, all metal connections being welded. The entire loop and the liquid metal used to fill it were treated with purified hydrogen at a temperature of 500 to 550 C. for at least one hour. Natural convection induced by a temperature diiferential was employed to circulate the metal. All loops were operated at a minimum temperature of 500 C. with the temperature differentials indicated in the examples between the hottest and coldest portions of the system.

Example I.Three separate loops were formed from 0.5 inch internal diameter tubing of steel containing 2.25% chromium and 1% molybdenum. Pure bismuth was circulated in each of these loops at a temperature differential of 40 C. until the formation of a plug consisting principally of iron and chromium prevented further circulation. The time of operation of the loops before plugging occured was 383, 300 and 260 hours respectively. The maximum penetration of the hot leg of the 300 hour loop was 2 mils.

Example II.Two loops having the same dimensions and composition as those employed in Example I were formed and bismuth containing 760 parts per million of uranium, 250 parts per million of magnesium and 250 parts per million of zirconium was circulated in the first and 880 parts per million of uranium, 1000 parts per million of magnesium and 1000 parts per million of zirconium was circulated in the other. A temperature differential of 40 C. existed in the loop. These loops were operated for 3000 and 2500 hours respectively without any signs of plugging.

Example III.--A loop for the circulation of metal was formed from steel tubing having a 1.3 inch internal diameter. This steel contained 2.25% chromium and 1% molybdenum, the same composition as employed in Example I. Bismuth containing 1000 parts per million of uranium, 250 parts per million of magnesium and 80 parts per million of zirconium dissolved therein was circulated in the loop, with a temperature differential of about 55 C. The circulation continued for 6,800 hours without the formation of a plug. Concentration of uranium remained constant while that of zirconium and magnesium decreased during the circulation. The maximum penetration of the hot leg was 3 mils.

Example I V.--A loop was formed of steel tubing having a 1.7 inch internal diameter. The steel contained 5% chromium and 1.25% silicon. Bismuth containing approximately 440 parts per million of uranium was circulated in the loop, with a temperature differential of 17 C. 'It was noted that the concentration of this uranium dropped continuously during the 2,441 hours of operation. After this period of operation the formation of a plug was indicated by an increase in temperature between the hot and cold legs and the circulation was stopped. Metallographic examination revealed that the loop was plugged in the coldest section. The material of the plug was found to contain uranium. Maximum penetration of the hot leg was 20 mils. 7

Example V.-A loop having the same dimensions and composition as that employed in Example IV was formed and bismuth containing 450 partsper'million of uranium, 350'partspermillion of magnesium and 250 parts per million of zirconium was circulated for approximately 9,000 hours,; with a temperature differential of about C. There were no apparent signs of plugging after th stime 1. Wher th on en a n of u u ha mlcmm during circulation of liquid bismuth containing uranium,

per million.

'4 the uranium concentration can be at least partially -re= stored according to the method of the subject invention by addition of inhibitors as illustrated in the following example.

Example VI.A loop of metal tubing having a 1.3 inch inside diameter was formed from steel containing 2.25% chromium and 1.5 silicon and bismuth containing 500 parts per million of uranium was circulated therein. A temperature diiierential of 30 C. existed and no inhibitors were added. Circulation by thermal convection was continued until the uranium concentration fell to 90 parts At this time 400 parts per million of magnesium and 250 parts per million of zirconium were added. The uranium concentration then rose to 330 parts per million where it remained during further circulation.

Frcrn the foregoing, it is evident that the subject method makes possible the very efiective inhibition of corrosion of available structural steels by bismuth.

It is also evident that the method has utility in at least partially reestablishing the concentration of uranium metal in liquid bismuth where the concentration has decreased during the circulation of the uranium solution in contact with steel.

Since many embodiments might be made of the present invention and since many changes might be made in the embodiment described, it is to be understood that the foregoingdescription is to be interpreted as illustrative only and not 'in a limiting sense.

We claim:

1. The method of inhibiting corrosion of a container vessel by high temperature bismuth flowing in said vessel, which comprises fabricating the vessel from a steel which contains between 2 and 12% chromium, between 0.5 and 1.5% of a metal selected from the group consisting of molybdenum and silicon, a minimum of nickel and manganeseand the remainder essentiallyiron and maintaining zirconium dissolved in the bismuth flowing therein at a concentration of between parts per million and the saturation concentration of zirconium at the lowest temperature of said vessel.

2. The method of preventing corrosion of vessels exposed to high temperature liquid bismuth containing uranium dissolved therein and for preventing the loss of uranium from solution in bismuth, which comprises fabricating the vessel from a steel which contains 2 to 12% chromium, between 0.5 and 1.5% of a metal selected from the group consisting of silicon and molybdenum,

a minimum of nickel and manganese and the remainder essentially iron, maintaining in said bismuth a zirconium concentratio n of between 80 parts per million and the saturation concentration of zirconiumat the lowest tem= q perature of the system and maintaining a magnesium concentrationof between 100 and 1000 parts per million.

3. The method of inhibiting the corrosionof containers by contact with liquidbismuth at a temperature above 400 C. which comprises fabricating said containers from a steel having between 2'and 1 2% chromium by weight,

- nesium in said bismuth of between 200 and 250 parts per million.

4. The method of restoring to solution in liquid bismuth at least a portion of the uranium which is lost from solution as said bismuth is circulated in a vessel formed from a steel containing between 2 and 12% chromium, between 0.5 and 1.5 of a metal selected from the group consisting of silicon and molybdenum, a minimum of nickel and manganese and the remainder essentially iron which comprises adding to the bismuth in said vesselv approximately 400 parts per million of magnesium and approximately 25 0 parts per million of zirconium.

5. As a high-temperature heat tra nsfer medium, bis: muth containing dissolved therein between 50 parts per million and the saturation value of zirconium in bismuth 7. The alloy of claim 6 containing also at least 100' at the lowest temperature of said medium.

6. An alloy for use as a liquid in a nuclear reactor comt r prising bismuth, uranium in a concentration up to the References Cited in the file 9 thls Patent maximum solubility of uranium in bismuth and concen- 5 Atherton et al.: Nuclear Engineering, Part II, .publ. tration of zirconium of between 50 parts per million and (1954) by American Institute of Chemical Engineers, the saturation value of zirconium in said alloy. Progress Symposium SeriestNo. 12,'vol 50, pp. 23-27.

7 parts per million of magnesium. 

6. AN ALLOY FOR USE AS A LIQUID IN A NUCLEAR REACTOR COMPRISING BISMUTH, URANIUM IN A CONCENTRATION UP TO THE MAXIMUM SOLUBILITY OF URANIUM IN BISMUTH AND CONCENTRATION OF ZIRCONIUM OF BETWEEN 50 PARTS PER MILLION AND THE SATURATION VALUE OF ZIRCONIUM IN SAID ALLOY. 